1. Field of the Invention
The present invention relates in general to a thermal transfer image printer and, particularly, to a signal processing system for compensating for nonlinearities and spectral response characteristics of the colored dyes used in the thermal transfer printing ribbon.
2. Description of the Prior Art
With the advent of video tape recorders and other sources of video signals such as portable video cameras and the like, it has become desirable to be able to produce a full-color, hard-copy print of the image displayed on a television receiver or video monitor. Additionally, there has been recently proposed an electronic still camera that does not require photographic film but records signals representing the image on a magnetic sheet, the magnetic sheet then being played back for visual display of the still image on a television receiver or video monitor. It is very desirable that this electronic still camera be able to produce a full-color, hard-copy print from the signals recorded on the magnetic record. Thus, there have been proposed thermal-transfer, color-image printers, in which a thermal head having a number of energizable heating elements is used in conjunction with a ribbon or transfer film having several primary colored dyes individually applied to it. This ribbon having the colored dyes on it is arranged between the thermal head and a sheet of paper or similar medium and, upon excitation of the thermal head, the heat causes the dye to be transferred from the ribbon onto the paper. By scanning the magnetically recorded image signals and moving the paper and the ribbon in a corresponding fashion a full-color, hard-copy print may be produced from the magnetically recorded image.
In this proposed thermal printer, the image signals are arranged as in a conventional television display and one vertical column of the recorded image is printed at a time, thereby requiring the thermal head to have a number of heat elements corresponding to the number of horizontal scan lines making up one frame of the recorded video signal. In order to have realistic results, the color print produced must have a number of shades or densities of the various colors and combinations of colors provided by the colored dye ribbon. Therefore, the gray scale levels of the colors making up the recorded image must be accurately transferred to the print. While the circuitry to accomplish this is known, a problem arises in that the response of each of the dye colors is not linear over the full range of densities necessary to produce a realistic print. That is, the response of each color dye is linear for only a portion of the density curve, this linear portion is usually used to determine the slope of the response curve and is typically referred to as the gamma curve of the dye. Accordingly, when producing a color print with the thermal head and colored dye transfer ribbon it is necessary to compensate for the non-linearities in the dye response, and this is typically called gamma compensation.
Various systems have been proposed to provide gamma compensation, that is, to account for nonlinearities in the density response of the various colored dyes employed in the thermal transfer ribbon, and a further problem is presented in that the density-versus-wavelength response of the three primary color dyes employed in the ribbon are not separate and distinct. That is, there are overlapping portions in the response curves of the three color dyes, and the result of this overlapping is that some colors will appear to be over-modulated or exaggerated, because when that color is printed to its maximum density a portion of one or both of the other colors is printed to a lesser extent. Thus, while gamma correction is known in thermal transfer color printer a further problem is presented in that no color correction relative to the color dye response is available.